Meg Tsiamis of Blogpond has written a couple of posts about StumbleUpon which I just happened to, well, stumble upon today.
People who've already used StumbleUpon will probably find her post on different ways to discover stuff to be useful, it talks about finding things randomly, through your friends network, and through keyword searches. The post ends with some tips on how to be a good user by making your Stumbles and discoveries count by accurately tagging them. There are also some helpful links to other articles.
She's also written a Beginners Guide to StumbleUpon.